The present invention relates to a spatial light modulator with regularly arranged pixels, where each pixel comprises a modulator element in the form of a controllable line grating for a complex modulation of a wave front, and to a method for realising this complex modulation.
Spatial light modulator devices comprise at least one spatial light modulator (SLM) which is provided based on micro-electro-mechanical systems (MEMS) in this invention. Various designs of MEMS-type SLM systems are known in the prior art under various names. Known embodiments are mirror arrays such as digital mirror devices (DMD), deformable mirrors (DM), piston micro mirror arrays, and diffraction-grating-based systems such as grating light valves (GLV), spatial optical modulators (SOM) or grating electro-mechanical systems (GEMS). Spatial light modulators are employed in a wide range of applications which are based on optical technologies and where variable or adaptive optical elements are preferably used.
The fields of application of spatial light modulators include display and projection systems for the consumer goods sector, microscopy (optical tweezers, phase filter, digital holographic microscopy, in-vivo imaging), beam and wave front forming using dynamic diffractive elements (laser material processing, measuring equipment, focus control), optical measuring equipment (digital holography, fringe projection, Shack-Hartmann sensor), and applications in maskless lithography, ultra-fast laser pulse modulation (dispersion compensation) or in terrestrial telescopes (dynamic aberration correction).
The pixels in the MEMS-based SLM systems are diffraction-grating-based modulator elements which employ the principle of the controllable diffraction efficiency in the reflected orders of phase gratings, where typically the ±1st orders are used for reasons of efficiency. The diffraction efficiency η of a diffractive element is generally defined as the quotient of the intensity of the exiting wave front and the intensity of the incident wave front. In a phase grating, the diffraction is realised by a phase shift which can be controlled either binary or continuously. Binary control requires a pulsed operation in order to adjust the desired greyscale value in the amplitude by way of pulse-width modulation. Embodiments of diffraction-grating-based MEMS-type SLM are known in which either the entire line gratings or individual elements of the gratings are moved vertically to achieve the modulation. Known diffraction-grating-based systems have in common that an amplitude modulation of the diffracted wave field is desired. The phase of the diffracted wave field cannot be modulated deliberately because it does not behave independent of the amplitude modulation on the one hand, and because it only varies slightly on the other.
In contrast, piston micro mirror arrays which deliberately only modulate the phase of the reflected wave field can be used as spatial light modulators. The phase is modulated in that adjacent pixels are given a mutual height offset, which causes a relative phase shift of the reflected wave field.
In many applications, an amplitude-only modulation, a phase-only modulation or the interrelated amplitude and phase modulation of a wave field as described above is sufficient. However, there are a number of applications where a complex modulation of a wave front is essential. A complex modulation means to set complex values with a real part and an imaginary part, i.e. here with amplitude and phase. Applications in which such a complex modulation is essential include holographic display systems, applications in optical information processing and data storage, and maskless lithography. The requirement of a complex modulation is reflected in the various documents which are concerned with these applications.
For example, encoding methods have been developed which also allow a complex amplitude of a wave field to be stored in phase-only or amplitude-only holograms. These methods, however, are at the cost of efficiency, resolution or phase reconstruction quality.
Documents EP 0 477 566 B1 and U.S. Pat. No. 7,227,687 B1 describe how a complex value is rendered in one pixel by way of combining multiple phase-shifting sub-pixels to get a large pixel, and how complex spatial light modulators can be made that way. Further, patent document U.S. Pat. No. 3,890,035 discloses combinations of multiple SLM, where the modulation of amplitude and phase is realised by two SLM which are arranged one after another.
It can be noted that in the most various fields of technology spatial light modulators are required to have the following characteristics: large number and small size of pixels (i.e. a large space bandwidth product), high modulation speed, great dynamic range, high diffraction efficiency, analogue or digital control with great accuracy and reproducibility, great fill factor, usability in various spectral ranges of the light and at various spectral densities.
It is known from the theory of diffraction-grating-based systems that both the diffraction efficiency (amplitude squared) and, to a minor degree, the phase will be affected if the relative distance between the grating and a basis is changed when modulating wave fields in reflection-type grating-based spatial light modulators. However, these two quantities are interrelated, i.e. cannot be controlled independently of each other. In order to be able to control the phase of the reflected wave fronts independently of the relative displacement of the grating normal to the modulator surface, a further degree of freedom of the movement of the gratings is necessary. It is known from interferometric measuring technology that a displacement of a line grating effects a phase shift in the diffraction orders m≠0. If a line grating is moved parallel to its grating vector and at right angles to the incident wave front, the phase of the wave which is diffracted in the mth order is shifted by m·2π times the number of grating periods p which move through a fixed reference point.
Theoretical background information which is relevant to understand the spatial light modulator according to this invention will be given in some detail at the end of the Description.